Method of lowering contrast of dye images



Patented Dec. 15, 1942 METHOD OF LOWE e tlx CONTRAST OF I DYE IMAGES Richard V. Young, Rochester, N. Y., assignor to Eastman Kodak Company, Rochester,

N. in, a

corporation of New Jersey No Drawing. Application August 31, 19%, Serial 1 Claim.

This invention relates to color photography and particularly to bleach baths for dye destructive processes of color photography.

Processes of color photography are well known in which a, uniformly dyed silver halide layer is exposed, developed, and treated with a bleaching agent which destroys the dye in the regions of the silver image formed in the layer. One of the objectionable features that has been encountered in some of these processes is that the contrast of the dye image produced by bleaching is too high. This is particularly true in those processes in which the bleaching of the dye takes place in an acid medium with the forma tion of a complex or insoluble compound of the silver. The diiiiculty with these processes is that the amount of dye bleached, which appears to be chemically equivalent to the silver that is oxidized to the complex or insoluble compound, has an optical density (for light of the color absorbed) which is many times as great as the optical density of the silver that is oxidized. Hence the gamma of the resulting dye image is many times the gamma of the silver image provided the bleaching action is allowed'to proceed nearly enough to completion to obtain clean highlights and an image with a sufficiently straight line characteristic curve in all layers of a multilayer coating.

It is, therefore, a principal object of the present invention to provide a bleaching bath for a film having a reducible dye uniformly dispersed therein which enables the contrast of the dye image to be controlled. Other objects will appear from the following description of my invention.

These objects are accomplished by adding to the bleach bath a mild oxidizing agent or hydrogen acceptor which competes with the imageforming dye during the bleaching action.

In dye bleaching processes of color photography, it is customary to add to a silver halide layer an image-forming dye which may be destroyed in the regions of a silver image by reduction. The dye may be added to the layer in its colored form or in the leuco form and the dye formed in the layer either before or after exposure of the layer. The layer is exposed and developedin the usual way to form a silver image and'is then treated in a bleaching solution which has a. preferential bleaching In Great Britain February 22,

action for the dye in the regions of the silver image. The bleaching agent may be one which is a reducing agent in itself but which reduces the dye only where silver is present as in the process described in Christenson U. S. Patent 1,517,049, November 25, 1924, or it may be one which bleaches the dye with the formation of a complex compound of the silver image.

, Especially in the second case, it has been found that the dye image produced by bleaching has excessive contrast in many cases and therefore does not constitute a true reproduction of the object photographed. It is not possible satisfactorily to control the contrast of the image merely by diluting the bleach bath or by bleaching for a shorter time. This tends to produce a distorted image since the bleaching may not be carried to completion in the desired regions and also is objectionable since in some cases the bleaching action is rendered exceedingly critical.

According to the method of the present invention, the bleach bath contains a mild oxidizing agent which successfully controls the contrast of the resulting dye image when used in proper amounts in the bleaching solution. In the bleaching reaction, the dye is reduced to a colorless form where it is acted on in the region of the silver image. In those processes where the image-forming dye is reduced by the reaction of the silver image and the bleach bath, the dye acts as an oxidizing agent with respect to the silver image, and, conversely, the silver image acts as a reducing agent for the dye. I propose to add to the bleach bath mild oxidizing agents which compete with this oxidizing action of the image-forming dye and decreases the chemical efficiency of the silver for reducing dye. The result is a lowering of the amount of imageforming dye reduced by each quantity of silver oxidized, andtherefore a lowering of the contrast of the resulting dye image.

The oxidizing agents which may be added to the bleach bath according to my invention include dyes such as Tartrazine, (Schulz No. 737), and Orange G, (Schulz No. 39), colored compounds such as 1-nitroanthraquinone-8sulfonic acid, organic compounds such as p-nitrosodiethylaniline, oor m-nitrobenzene sulfonic acid or inorganic compounds such as ammonium vanadate. and ferric ammonignthsulgilhatgbl'lfielsg I Ezampled compounds may be added e eac at mum 0000000 00000 00000 00 00000 23333.2?23333333ffiiiiIIIIIIIZ: 333:3 gram per 100 cc. of bleaching solution dep nd n Potassium bromide 3 upon the particular oxidizing ag nt an blea h mmethyl qumoxanne 03 bath used.

The following examples, which are illustrative mfifil gfggfifi "5" 3'2: only, indicate bleaching solutions which may be Water to 'i g used according to my invention: '7

0 Example 7 Example 1 Hydrochloric acid (cone) cc- 100.0 Hydrochloric acid (cone) --cc- 10 .0 sodium m n 1009 Sodium chloride 100-0 Potassium bromide ..-g 30.0 Potassium bromide Dimethyl quinoxaline g 0.3 Dlm t quinoxallne 03 Oxyamino phenazine g 002 oxyammo Phenazme o-Nitrobenzene sulionic acid g 8 T r w t m lifer 1 Water liter. 1 r

In these bleach formulas the dye is reduced Example 2 by the reaction 01 the silver image with the hy- Hydrochloric acid (conc.).- 100,0 drochloric acid, and the dimethyl quinoxaline Sodium chloride g 100.0 and oxyamino phenazine may be considered as Potassium mi g 30,0 catalyzing the bleaching reaction. Dimethyl quinoxaline g 0.3 I am aware that oxidizing agents have been Qxyamlno phenswine rz 0.02 added to bleaching baths that destroy azo dyes in m m ium ulfate g 1 the pressure of a silver image, but these oxidizw t t 11t r 1 ing agents have been used to destroy an image- Example 3 forming dye by oxidation in the presence of a silver image and have not been used in a bleach Hydrochloric acid (cone) -..cc-- 100.0 bath in which the image forming dye is destroyed Sodium chloride ....g- 100.0 by reduction, and hence have not been used to Potassium bromide g 30.0 compete with the image-forming dye for the re- Dimethyl quinoxaline g 0.3 ducing action of the silver image. Oxyamino phenazine ....g...' 0.02 My invention is susceptible of modification not p-Nitroso diethyl aniline g 0.2 herein specifically described and it is to be un- Water to liter 1 derstood that my invention is limited only by the Example 4 sciirzgariiilthe appended claim. y oc c 801d (00m!) 100-0 In a method of producing a. colored photo- Sodium chloride 8" -0 40 graphic image of moderate contrastin a gelatino Potassium bromide --8-- silver halide layer having a bleachable dye uni- Dimethyl q formly dispersed therein, the method of control- Oxyamino Phenazine --8-- ling the contrast of the dye image which comolange G (Schulz 39) --8-- 1 prises exposing and developing the layer to pro- Watel' 1 5 duce a metallic silver image therein, and treating Example 5 the layer with an acig bath that contains a reagent reacting with t e silver to bleach the dye Hydrochloric acid (conc') 100-0 and that also contains a relatively small amount Sodium chloride 100-0 of an inorganic metallic salt selected from the Pmassium bmmide group consisting of ammonium vanadate and f g gggfi g ggggggg "g" 3'3 ric ammonium sulfate, said salt being reducible 1 1tranthraquinne 8 sm1mc add-g 03 under the conditions of the bleaching.

Wa er to .liter.. 1

RICHARD V. YOUNG. 

